Quantum Dots (QDs) are man-made nanostructures, typically semiconductors, that usually vary from 1 nm to 10 nm in diameter. These dimensions are on the order of the De Broglie wavelength or Bohr radius of the exciton (electron-hole-pair) of the semiconductor material from which they are made. Electrons confined in these nanosized semiconductor structures exhibit electronic and optical characteristics similar to atoms. Like atoms, QDs are highly efficient light emitters with discrete narrow emission lines. The advantage is that their optical properties can be tailored. Their absorption/emission frequencies can be tuned by varying the semiconductive material as well as the size of the QD. QDs emit light as a result of the recombination of an electron-hole pair (exciton), and the size-dependent emission is a direct consequence of quantum confinement of the exciton due to the nanometer-scale size of the particles. In addition, their band gap can be varied by changing their size. As the dot gets smaller, the light they emit becomes shorter in wavelength (blue shifted) an, conversely, as they become larger the light they emit becomes longer in wavelength (red shifted). Hence, they absorb radiation of wavelengths above their band gap width and emit light over a very narrow wavelength (much like an atom). The peak emission wavelength is bell-shaped (Gaussian) and occurs at a slightly longer wavelength than the lowest energy exciton peak (the absorption onset).
Due to these important properties, QDs have been used extensively as contrasting agents for imaging in medical research, particularly for the molecular imaging of cancer cells (see, e.g., Bentolila, L. A. et al., Discovery Medicine, 5:26, 213-218; April 2005). Moreover, much research has also been applied to devise robust, versatile, and biocompatible (non-toxic) surface chemistries to both solubilize and functionalize nanocrystals for biological applications (see, e.g., Michalet, X. et al., Science, 307:538-544, 2005). Researches have also been able to use quantum dots to localize a tumor by conjugated the quantum dot with a tumor specific antigen (see, e.g., Xing, Y. et al., Nat. Protoc. 2:5, 1152-65, 2007; Kim, G. et al. Conf. Proc. IEEE Eng. Med. Biol. Soc. 1, 714-16, 2005; and Sinha, R. et al. Mol. Cancer Ther. 5:8, 1909-17, 2006).
Despite the advances made by those skilled in the art, several limitations exist with the current technology as well as several needs that remain unaddressed. Specifically, current technologies for the detection of cancer cells often use long wave ultraviolet light (or visible light) to excite the QD, thereby causing light emittance. Ultraviolet light of these wavelengths, however, is not able to penetrate into deep tissues, therefore tumors found deep within the body may not be detected. Further, there still exists a need to devise effective methods of treating cancers as well as infections with pathogens, particularly those which have become resistant to current methods of treatment (e.g. “superbugs” that are resistant to antibiotics, etc).
Thus it would be advantageous to provide compositions and methods to address these shortcomings of the prior art.